Neutralizing transformer arrangement



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ATTORNEY Patented July 8, 1930 HTED STATES PATENT OFFICE THOMAS SHAW,

OF HACKENSAGK, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE & TELEGRAPHCOMPANY, A CORPORATION OF NEXV YORK NEUTRALIZING TRANSFORIVKIERARRANGEMENT' Application led .Tuly 10, 1929. Serial No. 377,318.

This invention relates to transmission circuits, and more particularlyto arrangements for reducing external interference upon such circuits.

Where telephone or other communication circuits parallel high voltagecircuits such as power lines, the power lines introduce disturbingvoltages on the telephone circuits. It has been proposed to reduce thedisturbingJ voltages of the fundamental frequency by the use ofneutralizing transformers. This involves providing an auxiliary circuitin which similar disturbances are induced and providing neutralizingtransformers having primary windings in the auxiliary circuit andsecondary windings in the disturbed telephone circuits, the secondarywindings being so constructed and arranged that the voltages inducedtherein by the primary windings will tend to oppose those directly im'-posed by the power circuit.

Such-neutralizing transformers were used in telephone circuits forneutralizing intern ference to a limited extent at a time prior to thegrowth of present day operating practices, which involve an extensiveuse of telephone repeaters on a two-way basis in the voice frequencycircuits, and also the use of carrier systems on open wire lines. fUnder these essential practices, which are in common use throughout thetelephone plant, only a small' magnitude of impedance irregularities ispermissible and neutralizing transl former systems heretofore proposedhave involved impedance irregularities of such a character as to preventtheir use in modern telephone systems.

In this connection it will be noted that neutralizing transformer coilshave a considerable amount of mutual capacity and direct currentresistance and certain of the designs that have been used in the pastalso add an appreciable amount of inductance to telephone circuits. Inconsequence, the neutralizing transformer coils cause an objectionabletransmission loss in the telephone circuits in which they are used andalso cause substantial impedance irregularities. These impedanceirregularities would degrade the operation of the voice frequencytelephone repeaters, due to impaired balance between the line and thebalancing network circuits; they would impair transmission quality dueto distortion caused by reflections, which effects would be particularlyobjectionable in the individual channels in carrier lines; moreover,they would greatly increase the carrier frequency crosstalk, due toreflection. These various impairments would tend to be particularlyobjectionable in multi-transformer installations, and in specificinstances their cumulative effect would become intolerable.

In accordance with the present invention it is proposed to reduce thevarious telephone transmission impairments mentioned above by designingthe individual transformer coils so as to have closely similarelectrical characteristics to those of pieces of intermediate cablessuch as are frequently encountered in open wire lines, thus making itpossible to use loading in connection with these transformers inaccordance with practices now used in loading intermediate cables inopenwire lines. This concept imposes certain limits on the maximummutual capacity permissible in a neutralizing transformer coil betweenconductors of the same telephone circuit and requires the direct currentresistance to have a certain ideal ratio to the coil capacity, andfurthermore, makes it desirable to have practically a zero leakageinductance inthe associated telephone circuits. These generalrequirements can be approximated, so fary as the design of thetransformer is concerned, by using parallel winding arrangements for thetransformer coil to reduce the leakage inductance to associated telyephone circuits; by proper choice of conductor size so that theresistance will bear the right ratio to the coil capacity; and bysuitably sectionalizing the transformer coils where the coil mutualcapacity would otherwise be larger than the value which would fit inwith the standard loading arrangetion it is to be noted that in openwire lines used for carrier systems the high frequency transmissionrequires the `associated cable (or transformer) loading to have a highcutoff frequency which fact limits the mutual capacity allowed in atransformer coil to a much lower value than would be allowable whencarrier frequencies are not involved.

The effect of properly loading the transformer 'coils would be to giveto the coils substantially the same effective impedance as that of thelines with which they are associated, thereby reducing the impedanceirregularity and the consequent reflection effects. The transmissionlosses in 'the transformer coils would alsobe reduced due to thegeneraleffect of the loading treatment, which raises the transformerimpedance effective in the telephone circuits. y

The invention will now be more fully understood from the followingdescription when read in connection ,with the accompanying drawing inwhich Figure 1 is a schematic circuit diagram showing a typicalarrangement of a neutralizing transformersystem; Fig. 2

,shows a typical arrangement of the transformer coils on the core; Figs.3 and 4 show winding arrangements for the coils; Figs. 5 and fshow howthe transformer coils are built out and loaded in accordance with theinvention; and Fim 7,shows the application of theneutralizing;transformer to a loaded cableV circuit.

Neutralz'zz'ng transfowaers-geneml descriptime The neutralizingtransformer is a device for reducing'the magnitude of longitudinaldisturbing voltages of the fundamental frequency produced intelephonecircuits by electromagnetic induction from parallel powerdistribution systems. It has a plurality of windings corresponding tothe number of wires'in the `telephone line 0r cable which is exposed toelectromagnetic induction, these windings being grouped in coils to takecare of individualtelephone circuits, the coils for non-phantomedcircuits having two windings and those for phantom circuit groups havingfour windings.V For example, referring to Fig. 1, six non-phantomedpairs having conductors numbered1to8 andl? to 20, inclusive, areshown,each` wire including a winding forVV each neutralizing transformer ofwhich latter two are shown. The windings of each pair, as, for example,1v and 2, are

associated together inthe same coil as shown at CH. Twofp'hantomlgroupscomprising wires 9 to 12 and 13 to 16 are also shown. Each wire 4of thephantom group includes a winding forV "each 'neutralizingVV transformerand' thelwindings of1the wires ofa given phantom group, as, forexample,f9 to v12, are included inthe samev coil asshown at CM2. lThe,windingsof all coils are inserted in series with the telephone lines insuch a way that a maximum impedance is presented to induced longitudinalcurrents, that is, the mutual inductances between the windings of agiven coil aid the self inductances offered to induced longitudinalcurrents. Conse uently the mutual inductances oppose the sel inductancesand minimize the impedance offered to the circulating telephonecurrents.

T he transformer coils are divided into two groups of coils known,respectively, as primary and secondary coils, according to theirfunctions in the neutralizing transform- .er system, as illustrated inFig. 1. The prin Vmary coils, designteedClHS and Cun-20 1n Fig. 1, areinserted in the telephone wires that act as primary wires forneutralizing transformer system. In order to obtain optimum transformeraction between the primary and secondary coils with respect to theinduced longitudinal currents, it is desirable to distribute the primarycoils evenly and symmetrically on the transformer core, with respect tothe secondary coils. In the transformer design illustrated in Fig.2,`for use in a transformer system involving a total of four primarywires and sixteen secondary wires, two two-winding primary coils arerequired, one on each leg of the transformer core,`at the approximatecenter.

The primary wires for the neutralizing transformer system, in which theprimary coils are inserted, are grounded at each end of the section ofopen wire line or cable which is being neutralized by the transformer,as shown at G in Fig. 1. The induction currents flowing over the primarywires and through these primary transformer windings induce in thesecondaryV transformer windinjgs voltages which are in approximate phaseopposition to `the longitudinal voltages induced in the associatedsecondary wires by the disturbing power line. By proper design of thetransformers and the neutralizing transformer system, the longitudinalvoltages in the secondary wires can be reduced to be Aonly Ya smallfraction Aof the voltages which would act in the absence of theneutralizing transformers. Under these conditions, assumingapproximately 'complete neutralization, no current of power fundamentalfrequency iows inthe secondary windings and Y hence the neutralizingtransformer works on practically a no-load basis. As is known,

the effect ofa load on the'secondary of a transformer is to reduce theVimpedance looking Yinto the primary. Since the neutralizing transformeris operating effectively on a no; load basis, the impedance of theprimary windings to Vthe longitudinal currents is therefore-a maximum.This choke coil action of the primary transformer windings greatlyreducesthe magnitude of the longitudinal induction currents following inthe primary telephone wires. By desigi ing the transformers to maketheir primary impedance large with reference to the impedance of theprimary line wires and grounds, the longitudinal voltages in the primarywires of the neutralizing transformer system can be reduced to the sameorder of magnitude as the unneutralized longitudinal voltages in thesecondary wires.

The foregoing discussion is on the basis of using the primary wires ofthe neutralizing transformer system for telephone or other communicationpurposes. If the primary wires should have no other use than as primarywires for the neutralizing transformer system, the transformer primarycoil construction may be simplified from that illustrated in Fig. 2. Forinstance, it would not be necessary to have more than one primarywinding on each core leg and this would pref erably be distributedevenly over the entire core leg. The secondary coils, each having two orfour windings, depending as to whether non-phantomed or phanto-medtelephone circuits are involved, could be placed on the transformer coreconcentrically with the primary winding.

Impairment of telephone transmission performano@ As a considerablenumber of telephone circuits are necessarily brought together in eachneutralizing transformer, it is important to exercise great care in thedesign and manufacture of the transformers to avoid objectionablecrosstallr between the different tele.- phone circuits. This requires abalanced coupling among the windings of the telephone circuit coils. Forexample, consider conductor pairs 1 2 and 3-4. In order to avoidcrosstall, the windings of the coil @1 2 and CM should be so related andmounted on the core that the mutual inductance from concluetor l toconductor 3 will be the same as from conductor l to conductor 4. Alsothe mutual inductance from conductor a to conductor 3 should be thesaine as that from conductor 2 to conductor 4. These relations shouldalso hold with respect to capacitative coupling between the windings ofdifferent telephone circuits.

In order to bring about a substantial degree of balanced coupling,so'called parallel winding arrangements have been used in the past forthe transformer coils. The parallel wire winding arrangement for anon-phantom coil isshown in Fig. 3 and for a phantom group is shown inFig. 4. Taking the nonphantom pair 1 2 of Fig. l for purposes ofillustration, the two wires comprising the windings of the coil @1 2 arewound upon the coil form side by side as is shown in Fig. 3, and foreach succeeding layer outwardly coils, as above described, isadvantageous ,between conductors of the same telephone from the core ofthe two conductors, are reversed in position. This results in the individual parts of the two sets of wires comprising the coil beingsubstantially uniformly distributed throughout the cross-sectional areaof the coil with the result that the mutual inductance and the directcapacitance from an adjacent coil to any given conductor ofcoil @1-2,for example, will be substantially the same as that to the otherconductor of 7 the coil.

In the case of a phantom group, the four wires comprising the phantomgroup, as, for example, wires 9, l0, ll, and l2, are twisted togethereither in the twisted pair formation used in quadded cable or in thewell known spiral-four construction. The strand thus formed is thenwound about the coil form as though it were a single wire with theresult that the four individual wires of the quad are equally spacedthrough the cross-section of the coil. This also results in a goodcondition of magnetic balance.

While the parallel wire arrangement of the from the standpoint ofbalance so that the crosstallr is reduced to a minimum, the construction has an inherent disadvantage in that the coils have largemutual capacities circuit, due to their very close association with eachother.

The large mutual capacity of the transformer coils, together with theresistance effects, cause transmission losses in the telephone circuitsin which they are inserted, these losses being much greater for the highfrequencies involved in carrier telephone transmission than in voicefrequency transmission. By virtue of their large mutual capacity andassociated series resistance of their windings, the transformer coilsalso act as impedance discontinuities in the otherwise uniform telephonecircuit. Such impedance irregularities cause reflection effects whichwould (l) materially impair the operation of the telephone repeater inthe voice-- frequency channels of the carrier lines, and (2) causetransmission quality'distortion, and increase reflection crosstallr inthe carrier channels. The magnitude of these transmission loss andreflection effects is a function of impedance of the transformereffective in the telephone circuit, this impedance being a function ofthe magnitudes of the mutual capacity, series resistance and seriesreactance of the transformer coils. In well designed and properlyconstructed coils, the effect of the leakage conductance in determiningthe coil loss and impedance characteristics is negligible. Now, in anopen wire line with no transformer present, we have a uniform line withcharacteristic irnpedance dependent on distributed resistance, 130

inductance, and capacity in accordance With the formula R-I-L'Lw G -i-Ow Where w is nr times the frequency of the telephone current and Z, R,L, G, and C are, respectively the impedance, resistance, inductance,leakage conductance and capacity. Here also the factor G is not ofimportance at the frequencies with which We arefconcerned, and With thetype of line construction employed. `In general, the ratio of theconstants R, L and C of the neutralizing transformer coils Will bematerially different than the ratio of the corresponding series andshunt constants per unit length of the uniform line, so that an`impedance irregularity would result. Y

The various transmission loss and reflection effects above mentionedWould be objectionable for one transformer. If a considerable number oftransformers should have to be used in order to obtain a satisfactoryloW Y value of unneutralized longitudinal voltage in the exposedtelephone system, the net result of all of these transmissionimpairments could'reasonably be expectedin particular cases to make itimpossible to operate carrier systems over the neutralized open-'Wirelines, or to give satisfactory service with telephone repeaters, in thevoice frequency circuits.

Use of loading n conjmwtz'on with neutralzz'ng transformers and the sameimpedance characteristics as theline in which it is connected.

As previously noted, the parallel Wire` construction of neutralizingtransformer coils results ina considerable amount of mutual-capacity andseries resistance With a very low or practically zero series reactance.By

proper care in the design and construction of the transformer coils, inaccordance With the present invention the ratio of capacity toinductance to resistance may be made to approximate the correspondingratio in the standard types of intermediate cables. De`

pending uponthe degree of this approximation, vthe impedancecharacteristics andthe transmission loss characteristics of thetransformer coils can be improved by loading these coils in the same Waythat the impedance characteristics and transmission losses inintermediate cables in open-Wire lines are improved by the use ofstandard types of intermediate cable loading.

A-princip'aldesign requirement for such loading of intermediate cablesis to alter the impedance of the cables so that there Will be anegligible impedance discontinuity at the junctions of the cable andopen-Wire construction. In other Words, the characteristic impedance ofthe loaded intemnediate cable must be made equal to the characteristicimpedance of the open-Wire line over the entire frequency band involvedin the use of the circuit. These standard loading methods require theintroduction of lumped inductances at the ends of the intermediate cableor at some intermediate point or points), so thatthe ratio of inductanceto capacity per unit length in the intermediate cables will beeffectively the same as the ratio of inductance to capacity per unitlength in the open-Wire line. These loading coils are spaced at theproper intervals, to obtain a sufficiently high cut-off frequency, soVthat thecharacteristic impedance of the loaded cable Will beapproximately the same as that of the open Wire line throughout theentire frequency range normally involved in the use of the line. Also,in order to avoid impedance discontinuities at the loW voice-frequenciesWhere the conductor resistance is an important factor in determining thecharacteristic impedance of the circuit, it is desirable that the ratioof resistance to capacity per unit length in the loadedintermediatecables should be similar to the corresponding Vratio inthe associatedopen-Wire lines. This requirement is ordinarily met to a satisfactorydegree of approximation by choosing an optimum size of conductors in thecables. The existing standard loading systems and operating practicesresult in the fundamental impedance matching requirements being met to asatisfactorily close degree over the Widest frequency ranges noWinvolved in service over open-Wire circuits.

In the preceding paragraphs special emphasis has been placed on theloading design to meet the ideal impedance matching requirements. Anadditional important advantage of suoli loading is that the transmis- 'Ysion losses in the intermediate cables are also substantially reduced,approximately in the ratio of the impedance of the open-Wire line tothat of the (non-loaded) cable.

`In explanation of this effect, it may be stated that ingeneral theimpedance of nonloaded intermediate cable* is substantially VloWer thanthat ofthe associated open-Wire line, primarily because 'of the muchlarger ratio of capacity to inductance, per unit circuit length. In lowimpedance lines, the principal cause of attenuation loss is heatdissipation due to the relatively large current flow in the seriesYresistance of the circuit, this effect being proportional to the squareof the current iiowing and directly proportional to the magnitude of theresistance per unit length. The application of loading to such cablesraises the circuit impedance thereby reducing the magnitudev of thecurrent iiowing through the series resistance, and the magnitude of theheat losses which are proportional to the square of the series currents.

In accordance with the present invention, it is proposed to apply theforegoing principles used in connection with intermediate cable loadingto the neutralizing transformer coil, using preferably a coil of theparallel wire-wound type previously described. In applying these loadingprinciples to neutralizing' transformer coils, however, the use ofloading places certain dennite limitations on the magnitude of themutual capacity and the direct cu 1rent resistance which would bepermissible in a transformer coil in a given installation. These limits,of course, would depend on the frequency range involved in the use ofthe open-wire line and the characteristic impedance of the open-wirecircuit as determined by the size of the wires and the pin spacing usedin the open-wire line.

Uso of carrier loading fwz'th neutralizing transformer coils Forexample, it may be assumed that the neutralizing transformers are to beapplied to a 16e-mil open-wire non-pole pair circuit, with standard12-inch pin spacing, and that the superposed carrier systems requirefrequencies as high as 30 kc. to be eiiiciently transmitted on the sidecircuits of the phantom groups and that the line impedanceirregularities below this frequency are to be lrept at a small value.The nominal impedance of the side circuits will then be about 600 ohms.The standard @-4.1 compensated loading would, under these conditions, beused tov load side circuits of phantom groups in the entrance andintermediate cables in such open-wire lines. rIhs loading uses fullweight coils having 4.1 mh. inductance,

spaced at intervals of about 930 feet and is Y applied to cables havinga side circuit mutual capacity of 0.062 mf, per mile. rfhis cablemutual. capacity per loading section is about 10900 m-mf. The loadedsystem has a nominal cut-off frequency of about l5 kc. and a f) of about600 ohms.

In the use of standard C-4-1 loading in conjunction with neutralizingrtransformers on the type of line assumed in the preceding paragraph, theideal value of mutual capacity for the transformer coils would be 10900nominal impedance m-mf., that is, the standard cable loading sectionmutual capacity with which this loading was designed to work. In orderto obtain the optimum impedance matching characteristic at the lowspeech frequencies where the conductor resist-ance is a factor indetermining the characteristic impedance, each of the two line windingsof a transformer coil such as @1 2 of Fig. 1 should have a directcurrent resistance of about 1.8 ohms. Under these conditions, thetransformer coil would be substantially equivalent electrically to oneQ-foot loading section in standard No. 13 American wire gauge cable,this being the optimum type of cable (as regards impedancecharacteristics) for use at intermediate points with 16o-mil copperopen-wire lines.

Vhen standard C-Ll loading is applied to cables in open-wire lines,there are placed at each end of the cable compensated terminal loadingunits consisting of a fractional weight loading coil having about 4.0mh. inductance, shunted by an impedance compensator consisting of a.3120 m-mf. condenser in series with a 2.54 mh. inductance coil.` Thiscompensated 'loading termination has the property of adjusting thefrequency impedance characteristic of the loaded cable to be similar tothat of the associated open-wire line throughout the high frequencyrange involved in the use of the circuits. With ordinary half-coil orhalf-section loading terminations the impedance frequencycharacteristics of a loaded cable and a uniform line are substantiallydifferent at high frequencies even when the loaded cable and theopen-wire line have the same nominal impedance (J When there is only oneloading section involved in an intermediate cable loading installation,compensated loading units are pla-ced at each end of the cable. Fig. 5illustrates this particular application of C-Ll compensated loading to atwo-wire neutralizing transformer coil having a mutual capacityof 10900m-mf. and a resistance of 1.8 ohms per winding.

If the transformer coil should have a mutual capacity less than 10900m-mf. it can be built out to the desired value by introducing into thecircuit at one end of the coil a shunt condenser having an appropriatevalue of capacity. Also the resistance of the coil could obviously bebuilt out7 by adding noninductive resist-ences of proper magnitude inseries with each transformer winding.

However, if designv requirements set by neutralizing efficiency and costconsiderations should result in the transformer coil mutual capacitiesbeing in excess of the ideal value given above (for the illustrative eX-ample under discussion) ,additional difficulty would be involved inapplying the loading properly. One practical solution would be ofcapacity th an this, building out condensers can be used to adjust thecapacity to this ideal value, as discussed above.

In such sectionaliz'ation of neutralizing `transformer coils, it wouldbe necessary to construct the coils so that there would be noelectromagnetic or electrostatic coupling (with respect to telephonecurrents) between the coil sections; as otherwise the loading would notfunction properly according to design. This result may be accomplishedby making separateV lcoils of each half-section. For example, if a coilsuch as @1 2 in Fig. 2 is to be sectionalized, it would be made in theform of two separate coils each having two line windings. 1 Y

` By using well .balanced parallel wire construction in the transformercoils, as illustrated in Figs. 3 and 4, each section of the transformercoil Awould be .practically noninductive to the associated telephonecircuit, such, for example, as the pair 1 2 of Fig. l, and there wouldconsequently be practicalV ly zero magnetic coupling between the coilsections with respect to telephone circuits. (From the standpoint oflongitudinal currents in these section windings, however,

there would be a substantial electromagnetic coupling.) A simple way ofeliminating electrostatic coupling between the transf former coilsections would be to shield' each section from the others. For thispurpose sheets of copper -could vbe inserted between the coil sections.Suclishields should be constructed to prevent the shield from acting asshort-circuited turns on the transformer coil, and, as is well known,this `result can be prevented by arranging the ends of the sheet ofcopper acting as a shield so that they over-V lap each other withoutcontacting, this resulting in interrupting any possible conductivecircuit through the copper while affording a shielding effect.

The foregoing loading discussion has been in' terms of non-phantomedcircuits, or side circuits of phantom groups, since these are the typesof circuitsused in. high frequency carrier operation. In phantom groupapplications of neutralizing transformers, consideration would, ofcourse, have to be given also 4to loading the phantom circuits of thefourwire transformer coils. At present, the phantom circuits ofopen-wire lines are used only for voicee'frequency operation which factconsiderably simplifies the loading requirements for the phantomcircuits.V The voice-frequency phantom loading associated with the 04.1carrier loading, uses 12.8 mh. (full weight) loading coils spaced atintervals corresponding to a phantom circuit mutual capacity of 106,000m-mf. This is the capacity of 6 x9'30 feet of standard cable having0.062 mf. side circuit capacity per :mile and 0.100 mf. phantom circuitper mile.

weight loading coil at the middle of. a twosectioncoil, vas illustratedin Fig. 6. Here the neutralizing transformer is divided into twohalf-sections Cp and CD1, and side circuit loading coils S are includedin the side circuits between the sections, and the phantom loading coilP is included in the phantom between the sections. f

The inductance of the loading coil in this arrangement would have tobear a certain ratiozto the coil capacity, in order that the loadedtransformer coil would have the same nominal impedance as the associatedphantom open-wire circuit (MLP/0p). Compensating terminal loading unitssuch as shown at C in Fig. 6 may be provided as already described inconnection with Fig. 5. Obviously, instead of using one phantom loadingcoil at the center of the phantom transformer coil, as illustrated inFig. 6, it will be permissible to use two loading coils at the terminalsof the `transformer coil, each coil having onehalf `of the inductancethat would be the ideal value for a single loading coil used atthecenter of the transformer coils. Y Voice-frequency loading forneutralisz'ng transformer coils The problem of loading neutralizingtransformer coils occurring in open-wire lines lused exclusively forvoice-frequency transmission is less complex than the carrier loadingproblem above discussed, mainly because less severe electrical designrequirements are imposed onthe transformer coils. Although the availabledata on this point are limited,

to a high degree of probability it is indicated that any design oftransformer coil that would satisfy vthe voltage neutralizationrequirements would have a mutual capacity much less than that of aloading section in Vthe standard voice-frequency loading for entranceand intermediate cables in non- `loaded open-wire lines. On this basis,it would not be necessary to sectionalize the transformer coils andsatisfactory impedance results could be obtained in all cases by usingfractional weight coils at the terminals of the transformer coilcircuits, with or without building-out arrangements. There is apossibility Vthat theavailable fractional weight lib vsion impairments.

loading coils would not be the optimum coils for a particularVtransformer design. In such instances, it would not be difficult orunduly expensive to make available a suitable special loading coil. Thisoperation might be less expensive Lhan to use an available coil in thosecases where a large amount of building-out capacity would be required inorder to obtain satisfactory electrical results from the available coil.Problems of this kind are liable to be encountered occasionally inleading short intern'iediate cables in open-wire lines.

Transformer z'astalatons n loaded cable circa/ts A somewhat differentproblem from those considered above would be involved if a neutralizingtransformer should be installed in loaded cables operated in conjunctionwith telephone repeaters.

lf the transformer should be installed at the time the cable isinstalled, the loading coil spacing could be arranged to allow for thecapacity effects of the neutrali former coils. For instance, if the tmer coil capacity should be equivalent to that of a. quarter of astandard cable loading section capacity, the loading coils adjacenttothe neutralizing transformer could be spaced 25 per cent closer thanthe normal spacing. lf the mutual capacity of a complete transformercoil should be in excess of that of a standard cable loading section, itwould, of course, be desirable to sectionalize the coils, as discussedin the open-wire line application considered above.

lf neutralizing transformers should have to be installed on a cable thatis already loaded, it would be necessary to add extra loa;

' coils to the cable and to use capacity possibly also resistance)building-out arrangements in order to restore the regularity of the coilthe spacing'. Such an arrangement is illustrated in Fig. 7.

ln the foregoing discussion, no distinction has been made betweentransformer primary coils and transforn'ier secondary coils, withrespect to telephone transmission impairments, or means for reducing thetransmis- This follows from the fact that the primary coils would begenerally similar in design to the secondary coils in transformerinstallations where the primary wires of the neutralizing transformersystem are used for telephone purposes.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated, Without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

l. In a system for neutralizing induced voltages, a transmission circuitsubjected to induced voltages, an auxiliary circuit subjected to similarinduced voltages, a neutralizing transformer comprising a primary coilin the auxiliary circuit and a secondary coil in the transmissioncircuit, said coils each having a charactertistic impedance differingfrom that of the circuit in which it is included, and means to lead atleast one of said coils so that its impedance will be substantially thatof the circuit in which it is inserted.

2. n a system for neutralizing induced voltages, a. transmission circuitsubjected to induce-cL voltages, an auxiliary circuit subjected tosimilar induced voltages, a neutralizing transformer comprising aprimary coil in the auxiliary circuit d secondary coil in thetransmission circuit, said coils each having a chara'ctertisticimpedance differing from that the circuit in which it is included, andmeans to load at least one of said coils so that attenuation will bedecreased and its imice will be substantially that of the cirin which itis inserted. ln system for neutralizing induced transnfiission circuitsubjected to d voltages, an auxiliary circuit subjcctc`L to sinilarinduced voltages, a neutralqnier comprising a primary coil y circuit anda secondary coil in ie transmission circuit, said coils each havi gacharacte tic impedance differing from Aiat of the cncuit in which it isincluded, means to build out at least one of said coils, and means toload any coil which is built out so 'that its impedance will besubstantially the saine as that of the circuit in which it is inserted.

4. In a l system for neutralizing induced voltages, a transmissioncircuit subjected to induced voltages, an auxiliary circuit subjected tosimilar induced voltages, a neutralizing transformer comprising aprimary coil in the auxiliary circuit and a secondary coil inthetransmission circuit, said coils each having a characteristic impedancediffering` from that of the circuit in which it is included, means tobuild out at least one of said coils, and means to load any coil whichis builtout so that its attenuation will be decreased and its impedancewill be substantially the same as that of the circuit in which it isinserted.y

5. In a system for neutralizing induced vol-tages, Va transmissioncircuit subjected to induced voltages, an auxiliary circuit subjected tosimilar induced voltages, a neutralizing transformer comprising aprimary coil in the auxiliary circuit and a secondary coil in thetransmission circuit, said coils each` its ratio of inductance tocapacity to resistance Will be similar -to that of the circuit in Whichit is inserted.A

6. In a system for neutralizing induced voltages, a transmission circuitsubjected to induced voltages,`an auxiliary circuit subjected to similarinduced voltages, a neutralizing transformer comprising a primary coilin the auxiliary circuit andl a secondary coil in the transmissioncircuit, said coils each having a characteristic impedance differingfrom that of the circuit in which it is included, means to build out thecapacity of vat least one ofsaid coils so that its ratio .in thetransmission circuit, said coils eac-h having a characteristic impedancediering from that of the circuit in Which it is included, at least oneof said coils alsobeing divided into sections each having a capacity notexceeding that of a normal cable loading section, means to build outeach section so that the ratio of capacity to resistance Will be similarto that of the circuit in which the section is inserted, vand means toload the section so that the ratio of capacity to resistance toinductance Will be similar to that of the circuit in Awhich they areVinserted.

8. In a system for neutralizing induced volta-ges, a transmissioncircuitl subjected to induced voltages, an auxiliary circuit subjectedto similar induced voltages, a neutralizing transformer comprising aprimary coil in the auxiliary circuit anda secondary coil,

in the transmission circuit, said coils each having acharacteristlcimpedance differing from thatof the circuitin which it isineluded, at least one ofsaid coils also being divided. into sectionseach having a capacity not exceeding that of a normal cable loadingsection, meansto build out each section so that the ratio of capacity toresistance will be` similar to that of the circuit in which it isinserted, and means to load the sections'so that the ratio of capacityto resistance to inductance Willl be such as to produce an impedanceequivalent to that of the circuit in which it is inserted.`

9. In a system for neutralizing induced voltages, a transmission'circuit subjected to induced voltages, an auxiliary circuit subjectedtosimilar` induced voltages, a neutralizing` transformer comprising a.primary coil in the auxiliary circuit and a secondary coil in thetransmission` circuit, said coils each having'V a characteristicimpedance differing from that of the circuit in Which it is included,and means to load at least one of said coils so that its impedance Willbe substantially that of the circuit in Which it is inserted, and sothat the cut-off frequency of the loaded transformer coil Will besuiiiciently high to permit efficient transmission of the highestfrequencies involved in the use of the transmission circuit.

10. In a system for neutralizing induced voltages, a loaded transmissionsystem exposed to induced voltage, a neutralizing transformer having acoilin the loaded transmission circuit, means for adjusting theimpedance of the transformer coils by modifying its series resistance,shunt capaci-ty and series inductance to have the same ratio ofresistance to capacity to inductance and the same cut-off frequency asthat of the loaded transmission system.

In testimony whereof, I have signed my name to this specification this5th day of July 1929.

THOMAS SHAW.

